Aluminum hydroxide (also known as alumina trihydrate) is used to produce primary aluminum metal using electrolytic reduction processes. Aluminum hydroxide is produced on an industrial scale by a well-established procedure known as the Bayer Process. In this process, the steps involving crystallization and precipitation of solubilized aluminum hydroxide values from process liquors, are critical relative to the economic recovery of aluminum values. Economic recovery is realized by optimization of two commercially significant parameters: yield and average particle size.
Efforts to increase the yield and particle size of the aluminum hydroxide recovered from Bayer process liquor have failed to provide optimization of a commercially significant product. With the steep rise in energy costs during the past few years, aluminum hydroxide processing operators continue to seek optimal parameters for producing a commercially-acceptable aluminum hydroxide product. Despite efforts to identify chemical additives and methods to reduce the number of undersized alumina particles while maintaining process efficiency, none have been entirely successful in increasing yield and particle size of the alumina recovered in Bayer process liquors. The products of these efforts include filter aids such as coagulants or flocculants optionally containing polyacrylic acids (PAA), crystallization modifiers, and dewatering agents. For example, U.S. Pat. No. 4,737,352 (hereinafter the '352 patent) assigned to Nalco discloses a method providing a reduced percent of small size crystals and an increase in the yield of coarser aluminum hydroxide crystals by adding a blend of surfactant dispersed in oil to the pregnant liquor during the precipitation phase of the process.
The claims in the patent limited the surfactant to a tall fatty acid. The specification of the '352 patent, however, disclosed the surfactant as any fatty acid having at least a saturated or unsaturated four carbon alkyl backbone optionally containing at least one functional group. Additionally, the specification discloses a functionalized C8 alkyl fatty acid as advantageous; the fatty acid was not claimed and the advantages of this fatty acid is not disclosed or taught by actual or constructive reduction to practice. The specification defines the improved method as treating hot caustic Bayer process green liquor with a surfactant/oil blend. The specification does not teach, describe, or remotely suggest that the length of the carbon chain of the fatty acid is a factor imparting novelty. Two commercial crystal growth modifiers having fatty acid chains of greater than ten carbons were commercialized and are referred to as Commercial Product 1 and Commercial Product 2 in examples exemplifying the inventive step of teaching fatty acid chain length in improved aluminum hydroxide production. The '352 parent discloses a genus of fatty acids in oil that result in an improved commercial product. The fatty acid composition described herein is directed to a fatty acid species imparting unexpected results from those described in the genus patent.
The limitations of yield and particle size of alumina recovered from Bayer process liquors are also disclosed in U.S. Pat. No. 6,168,767 (hereinafter the '767 patent) entitled “Production of Alumina” assigned to Ciba Specialty Chemicals Water Treatments Limited. A water-soluble crystallization modifier formulation is disclosed comprising a first composition of a polyalkoxylated non-ionic surfactant and a second composition comprising a surfactant, or a precursor thereof, which is not polyalkoxylated. Ethylene oxide (EO) units are identified as essential components of the formulation in the polyalkoxylated non-ionic surfactant, preferably, ethylene oxide and propylene oxide (PO) units which form an ethylene oxide-propylene oxide block copolymer. The '767 patent discloses a composition which contains “substantially no mineral oil or silicone oil” and emphasizes regularly that the “advantage of the crystallization modifiers . . . is that they do not require the presence of oils.” (e.g., see column 2, lines 21-25; col. 4, lines 25-35; col. 5, lines 21-33). The cost effectiveness of these components and their acceptance when compared to the surfactant/oil blends used in the majority of crystallization modifier formulations in most Bayer processing plants today remains questionable.
Also affecting the particle size and product yield parameters in alumina recovery is the presence of oxalate in the pregnant liquor. Oxalate is a contaminate that can act as a seed site resulting in too many small hydroxide crystals, thereby lowering average particle size. Further, screening processes intended to remove oxalate impurities also remove alumina, thereby reducing product yield. These phenomena must be considered in any recovery process.
Despite efforts to satisfy the demands made by continuous and ongoing development of the Bayer process worldwide, the industry needs for an improved alumina product remain unfulfilled.